Glow plugs are known, for example, from published German Patent Application No. DE 28 02 625. Such a sheathed-element glow plug includes a tubular metallic housing which bears a thread on its outer circumference, by way of which the sheathed-element glow plug is screwed into the cylinder. At the end of the housing of the sheathed-element glow plug nearest to the combustion chamber, a glow element is enclosed by the housing and cantilevered so that it reaches towards a sheathed-element glow plug built into the engine. A heating device is arranged in the glow element which, at the combustion chamber end, makes contact with the closed bottom of the glow element to make a ground connection, and at the end away from the combustion chamber makes contact with the supply voltage via a contact stud. Ceramic glow plugs are also known, in which the part reaching into the combustion chamber is made of ceramic. In the known glow systems, the current through the heating device is switched on or off by a preheating time control unit via a switch (relay, power transistor).